Internet Networking recitation #1 Subnet + CIDR
Winter Semester 2013, Dept. of Computer Science, Technion
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Administrative Information
Course site: webcourse.cs.technion.ac.il/236341
Grading policy:
15% homeworks + 85% final exam 6 home assignments. Submission is in pairs.
Teaching Assistants:
Ran Koretzki • Office location: Taub 211 • Teaching hours: Tuesday 12:30 – 13:30 • Reception hour: Tuesday 10:30 – 9:30 • Email :
[email protected] Adi Omari • Office location: Taub 700 • Teaching hours: Monday 16:30 – 17:30 • Reception hour: TBD • Email :
[email protected] Internet Networking 236341
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IP Addressing: Original Classful Scheme
IP Address – 32-bit integer globally unique address
Dotted Notation: 132.68.37.54
IP Classes – dividing an address to net id and host id
The prefix (net id) identifies a network. The suffix (host id) identifies a host on this network.
Internet Networking 236341
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IP Addressing: Original Classful Scheme
Class A – 7 bits to net id, 24 bits to host id 1.0.0.0 – 126.0.0.0 Class B – 14 bits to net id, 16 bits to host id 128.0.0.0 – 191.255.0.0 Class C – 21 bits to net id, 8 bits to host id 192.0.0.0 – 223.255.255.0 Class D – for multicasting Class E – reserved for future use (used for private addresses)
Weakness
Growth of routing tables in routers Tens of thousands small (class C) networks. Each network must be advertised. Inflexible Lack of a network classes for mid-sized organization (between class B and C). Address space will be eventually exhausted Internet Networking 236341
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Subnet Addressing
A site has a single IP network address assigned to it, but has two or more physical networks. Different technologies. Limits of technologies. Network congestion. Security consideration. • VLAN – separate one physical network into a few logical networks. Administration (e.g. different departments in academic institute).
From outside it looks like a single network Only local routers know about multiple physical networks inside and how to route traffic among them Host ID is divided into a subnet ID and host ID Accepted as a standard at 1985 (RFC 950).
Internet Networking 236341
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Subnet Routing
When a router gets a packet, it isolates by Net mask the packet net id address. Each routing entry contains a net mask. Routing is done on a longest-match basis. If the packet is destined to other network then the router sends it to another router.
Otherwise the router sends the packet to the appropriate host on its attached networks.
Internet Networking 236341
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Subnetting - Example
Network 128.10.1.0/24 128.10.1.1
Rest of the Internet
H1
128.10.1.2
H2
R .
Network 128.10.2.0/24 All traffic to 128.10.0.0/16
H3
128.10.2.1
H4
128.10.2.2
A site with two physical networks. Using subnetting, R advertise these networks as a single network (thus, R accepts all traffic for net 128.10.0.0) Internal routing is done according to subnet id (i.e. the third octet of the address). Internet Networking 236341
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Variable-Length Subnetting
Motivation: Consider the case when an organization has a few networks of different sizes.
When we choose the subnet partitioning, we actually define constant number of possible physical subnetworks with maximum number of hosts on them.
Difficult to keep small (waist of subnet numbers) and big (the host id needs more bits) sub networks and there could be unnecessary spending of address space.
Solution: Variable-Length Subnetting. A subnet partition is selected on a per-network basis.
Internet Networking 236341
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Example – Configuring a Network with Variable-Length Subnetting
We have a network with IP 202.128.236.0/24
We need to support next sub networks:
6 networks with 26 hosts 3 networks with 10 hosts 4 networks with 2 hosts
If we take subnet mask of /27 bits then we can get 8 sub networks of 30 hosts (all 0’s and all 1’s of host addresses are reserved).
11111111.11111111.11111111.11100000 We need only 6 such sub networks.
The rest 2 sub networks we will partition by subnet mask of /28 bits.
11111111.11111111.11111111.11110000 We will get 4 sub networks of 14 hosts in each
We need only 3 such sub networks. Internet Networking 236341
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Example – Configuring a Network with Variable-Length Subnetting
The rest we will partition by subnet mask of /30 bits.
11111111.11111111.11111111.11111100
We will get 4 sub networks of 2 hosts in each.
Subnet mask #1 = /27
Subnet mask #2 = /28
11111111.11111111.11111111.11100000
11111111.11111111.11111111.11110000
Subnet mask #3 = /30
11111111.11111111.11111111.11111100
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Example – Configuring a Network with Variable-Length Subnetting Network address size
Subnet
port
202.128.236.0
2^5-2=30 hosts
255.255.255.224
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202.128.236.32
2^5-2=30 hosts
255.255.255.224
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202.128.236.64
2^5-2=30 hosts
255.255.255.224
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202.128.236.96
2^5-2=30 hosts
255.255.255.224
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202.128.236.128
2^5-2=30 hosts
255.255.255.224
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202.128.236.160
2^5-2=30 hosts
255.255.255.224
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202.128.236.192
2^4-2=14 hosts
255.255.255.240
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202.128.236.208
2^4-2=14 hosts
255.255.255. 240
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202.128.236.224
2^4-2=14 hosts
255.255.255. 240
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202.128.236.240
2^2-2=2 hosts
255.255.255.252
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202.128.236.244
2^2-2=2 hosts
255.255.255.252
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202.128.236.248
2^2-2=2 hosts
255.255.255.252
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202.128.236.248
2^2-2=2 hosts
255.255.255.252
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Internet Networking
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Classless Inter-Domain Routing (CIDR) - RFC 1519
Routing destinations are represented by network and mask pairs. Enabling network aggregation; thereby reducing the size of routing table.
Examples: Class A networks are followed by a /8 Class C networks are followed by a /24 8 Class C hosts network is followed by /21 Such a network has 21 bits of Net-ID, 11 Bits of Host-ID Contains 2^21 Net IDs, and 2^11-2 = Hosts in Each network.
Internet Networking 236341
